The Sap Flow Characteristics Of Larix Principis-Rupprechtii And Influencing Factors In Small Watershed Of Diediegou, Liupan Mountains, Ningxia, China

Larix principis-rupprechtu is one of the main afforestation tree species in North China. To explore the transpiration characteristics of Larix principis-rupprechtii and the relationships with environmental factors in different time scales and different slope positions, continuous monitors of the time scales and slope positions changing were focused on respectively during the growing season of 2014 and 2015 in watershed of Diediegou, Liupan Mountains, Ningxia.The results show that:In the hourly scale, the sap flow velocity’s changing appears a regular characteristic-increase first and then decrease during typical sunny day, the sap flow velocity’s difference is obvious when changing in the cloudy and sunny and rainy days and sap flow’s starting time, rising time, reducing time and the maximum are different. The peak size is as follow:middle stage is greater than the early than the later. The change of the sap flow velocity has the same type in hourly scale in different months, but the sap flow’s starting time, rising time, reducing time and the maximum are different. The stem sap flow’s changing is obvious during the early and the middle growing season from 5 to 8 month, about 0.15ml·cm-2·min-1, quickly reducing during the late stage of September, becoming very weakness at the end of the growing season, about 0.15ml·cm-2·min-1.Responsing to the environmental factors solar radiation has the same change trend with sap flow velocity, but the presence of the peak is earlier than sap flow velocity about 1 hour. Saturated vapor pressure deficit and air temperature have the similar change characteristics with the sap flow velocity, but the presence of the peak is later than sap flow velocity. Air relative humidity has the instead of law to saturated vapor pressure deficit, the opposite trend with sap flow velocity. the peak of Soil temperature presence about 10 hours later than the peak of sap flow velocity, Soil water content’s change is small and the relationship with sap flow velocity is weak in hourly scale.In the daily scale, changes of solar radiation, air temperature, the saturated vapor pressure deficit and relative humidity of air are very intense, especially during in cloudy and sunny day, soil temperature’s change is very gentle, less volatile in daily scale, soil water content will decrease with the absence of precipitation and rise rapidly when it rains. Sap flow velocity does not have regular volatility during the cloudy and sunny days. Analyzing of daily scale of different months, sap flow velocity changes in the middle of the growing season> early stage in growing season> late stage in growing season.In the monthly scale, the relevant environmental factors of solar radiation intensity and saturation vapor pressure deficit show a trend of gradually reduce. Air temperature, soil temperature and air relative humidity increase at the early stage time, and then decrease later, the first maximum is 17.74℃ appearing in July and the second maximum is 13.45℃ appearing in August, the latter appears in September about 55.48%.Soil water content decreases and then increase, the minimum is 32.59% appearing in June. Sap flow velocity presents highly in pri-mid stage and lower in mid-lat stage, the maximum in July is 0.076ml·cm-2·min-1, the minimum in October is 0.0008ml·cm-2·min-1.Correlation analysis between sap flow velocity and environmental factors show that the relation is highly significant with meteorological factors, soil water content in hourly and daily scales. But the dependency reduces with influencing factors besides the saturated vapor pressure deficit. Regression analysis between sap flow velocity and environmental factors show:environmental factors filtering into regression equation are change with different time scales and months, the main environmental factors affecting the sap flow velocity have time scale difference. Saturated vapor pressure deficit, solar radiation intensity, relative humidity of air and soil temperature are the main influencing factors in hourly and daily scales. The role of soil temperature in daily scale stronger than the hourly scale, solar radiation intensity in hourly scale stronger than the daily scale, saturated vapor pressure deficit is the most important factor influencing the sap flow velocity.The relationship between the sap flow velocity and environmental factors has obvious slope difference. During the variation of environmental factors in different positions, the soil water potential size is as follows:lower slope> middle slope>upper slope; changing rate of soil water potential is as follows:upper slope> middle slope>lower slope; sap flow velocity’s discrepancy changes from upper slope> middle slope>lower slope of the early stage to lower slop> middle slope> upper slope of the later stage. Soil water potential affects the position discrepancy of sap flow velocity. The difference is obvious when the order of precipitation before and after rainfall is different, the precipitation the order of 30 mm can significantly improve the sap flow velocity peak of all slope positions, the upper and middle slopes’ supply is obvious, but the supply’s time is the longest in lower slope position. Correlation and regression analysis in the slope between sap flow velocity and environmental factors show that:correlation of soil water potential is as follow:upper slope> middle slope > lower slope, the other meteorological factors are as follow:lower slope> middle slope> upper slope. Soil water potential is the determining factor in upper and middle slope, its effect gradually diminish from upper to lower, solar radiation intensity’s effect gradually enhancefrom upper to lower.